Mobile communication is becoming increasingly popular and the recent advances in digital processing have allowed a rapid migration of mobile services from analog communications to digital communications. Increasingly, development efforts are focusing on techniques for high-capacity communication of digital information over wireless links, and much of this wireless development work incorporates spread-spectrum communications technology.
Spread-spectrum is a method of modulation that spreads a data signal for transmission over a bandwidth, which substantially exceeds the data transfer rate. Direct sequence spread-spectrum involves modulating a data signal onto a pseudo-random chip sequence. The chip sequence is the spreading code sequence for spreading the data over a broad band of the spectrum. The spread-spectrum signal is transmitted as a radio wave over a communications media to a receiver. The receiver despreads the signal to recover the information data. The evolution of wireless communications to very high data rates with packet transmissions over the air has imposed constraints on the receiving system radio-frequency (RF) stages as well as on the operation of the analog-to-digital converters used therein.
Due to large variations in received signal power caused by propagation attenuation (e.g., fading due to man-made objects such as buildings or natural terrain features such as hills), a control mechanism is often used to dynamically control the gain of the receiving amplifier so that subsequent radio-frequency (RF) sections and digital sections of the receiving system can operate within a desired operating range. These sections include amplifiers, mixers, analog-to-digital converters and baseband analog or digital processing devices. The control mechanism for adjusting the amplification of the received signal level is referred to as automatic gain control (AGC).
An AGC circuit is designed to keep the amplified received signal at a near-constant power level over a large range of received signal levels. Three parameters involved in designing an AGC circuit include its operational range, its response time, and its steady-state error. The operational range of an AGC circuit in current spread-spectrum communications systems can easily exceed 80 to 90 dB in signal power. Some conditions that can contribute to this wide operating range include signal attenuation caused by hills or buildings and power control failure occurring when a mobile transmitter is in close proximity to a base station receiver.
Normally in dynamic control systems, the response time of the system is inversely related to its steady state error. As the control system parameters are configured and set to improve the system's response time, system instability, such as overshoot conditions, are likely to increase. In high data rate digital communications, and especially in packet switched systems, the conflict between these last two design parameters becomes increasingly important. In these types of systems, the data transmission interval can be as small as a fraction of a millisecond and even shorter. Because the start of each packet introduces a large signal variation and the periods of symbols within a packet are so short, a conventional AGC is not able to ensure timely amplification control for the received signal and, therefore, will prove ineffective at providing reliable data communications. Under these circumstances, a need, unmet by conventional AGC circuitry, exists for an AGC circuit that can quickly adjust the gain of a received signal during only a small period of time after a large received signal power fluctuation and can also provide smooth and stabilized operation during the remainder of data reception.